The Obama administration sharply criticized Chinese President Xi Jinping on Wednesday after charging that China’s military had deployed batteries of advanced missiles on a disputed South China Sea island.

Secretary of State John Kerry said the missile deployment was at odds with a pledge made by Mr. Xi while visiting the White House last year to refrain from militarizing clusters of disputed islands throughout the South China Sea.

Chinese officials, while not directly confirming or denying U.S. and Taiwanese claims about the missiles, said they plan to keep strengthening defensive capabilities in the South China Sea.

While both sides have no appetite for open conflict, their entrenched positions are narrowing their options for a diplomatic solution, while fueling the potential for dangerous flare-ups.

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U.S. officials said Wednesday that they had confirmed commercial satellite imagery appearing to show the deployment of surface-to-air missiles on Woody Island in the disputed Paracel chain. The commercial images, from the firm ImageSat International NV, indicate the missiles were deployed sometime after Feb. 3.

The U.S., which in recent months challenged Beijing by sailing warships and flying bombers near Chinese-held islands, had warned against China’s buildup in a region where both sides increasingly find themselves locked into opposing positions.

“When President Xi was here in Washington, he stood in the Rose Garden with President Obama and said China will not militarize in the South China Sea,” Mr. Kerry said on Wednesday. “But there is every evidence, every day that there has been an increase of militarization of one kind or another. It’s of serious concern.”